Technical Field
The present invention relates to a switching power supply that maintains a prescribed overcurrent protection level regardless of fluctuations in input voltage.
Background Art
Quasi-resonant switching power supplies are equipped with a switching element connected in series to the primary coil of a transformer to which a DC input voltage is applied. This switching element turns the DC input voltage ON and OFF. Examples of such a switching element include IGBT devices and power MOSFETs. These types of switching power supplies are also equipped with a rectifying and smoothing circuit that rectifies the voltage induced in the secondary coil of the transformer to generate a DC output voltage. The switching element is turned ON and OFF according to this DC output voltage.
The control circuit that controls the switching element typically turns the switching element OFF when the current flowing while the switching element is ON reaches a certain feedback control value calculated according to the DC output voltage. Then, when the current flowing in the transformer while the switching element is OFF inverts, the control circuit turns the switching element back ON to achieve quasi-resonance in the current flowing through the transformer.
A control circuit that turns the switching element ON and OFF in this manner is known as a quasi-resonant control circuit. As the switching element is repeatedly turned ON and OFF, the current that flows through the primary coil of the transformer induces a voltage in the secondary coil of the transformer, and the rectifying and smoothing circuit generates a DC output voltage.
This type of control circuit (a quasi-resonant control circuit) typically includes an overcurrent protection circuit that limits the peak current that can flow through the switching element. As disclosed in Patent Document 1, for example, this overcurrent protection circuit measures a current detection signal that corresponds to the current flowing through the switching element when the switching element is turned ON. When the current detection signal exceeds a predetermined threshold current value, the overcurrent protection circuit determines that the switching element is in an overcurrent state and forcibly turns OFF the switching element to protect the switching element from that overcurrent.
Patent Document 1 also discloses a method for improving the dependence of the overcurrent protection circuit on the DC input voltage. In this method, a pulse-shaped forward voltage that is proportional to the DC input voltage and is induced in the auxiliary coil of the transformer when the switching element is ON is detected, and the threshold current value for overcurrent protection is changed according to the detected forward voltage. More specifically, the dependence of the overcurrent protection feature on changes in the DC input voltage is reduced by implementing an input correction control scheme in which the higher the DC input voltage becomes, the lower the threshold current value is set.